In watching sports, such as soccer or rugby, which are played in a large playground by players who move quickly, or in watching a play in a theater having a laterally extending stage, it is desirable to use a monocular or binoculars having a wide view.
As to an observing unit incorporating a telescopic optical system to be used for these purposes, a type of binoculars called "opera glasses" has been widely used. Opera glasses are formed of the Galileo optical system, as known in the art, which uses a convergent lens as an objective lens and a divergent lens as an eye lens. The Galileo optical system advantageously allows opera glasses to be small in size and lightweight to be used conveniently, but results in opera glasses having a relatively narrow view, a dark field of view, and a low power.
On the other hand, a high-power observing unit incorporating a telescopic optical system includes a reverse reflection optical system, which is composed of two or three prisms placed between an objective lens and an eye lens. The overall optical system of such an observing unit is complicated. Thus, although this type of an observing unit may have a power of up to seven magnifications, its actual view tends to be narrow, for example, less than 10 degrees. Accordingly, this type of observing unit has not been adequate for observing an object that moves quickly.
In an observing unit incorporating a telescopic optical system, such as binoculars having a high power, a reverse reflection optical system has been the hindrance to obtain a wide view. Accordingly, the overall design of a telescopic optical system itself has been the key to obtaining a wide view. As a result, the arrangement of a telescopic optical system has become complicated, greatly increasing the weight, size, and cost of the observing unit that incorporates such a telescopic optical system, and further negatively affecting its portability and manipulatability.
In general, although an antinomic condition that the higher the power, the narrower the view cannot be avoided in principle, it is known in the art that using an objective lens having a minimum focal distance is preferable in obtaining a wide view. On the other hand, if a reverse reflection optical system composed of two or three prisms is incorporated in an image-erecting optical system, an objective lens should have a focal distance that takes into consideration the optical path length of these prisms. Accordingly, the overall optical system becomes bulky in proportion to the size of the prisms. This makes it difficult to form compact and, thus, portable observing units. Further, the size of a reverse reflection optical system comprising a couple of prisms determines the size of an objective lens to be used.
A need exists for a compact monocular or binoculars having a wide view, a bright field of view, and a high power.